The present invention relates to communications devices and protocols such as those used in wireless alarm systems having multiple sensors in communication with one or more receiver/controller units; and in particular to such alarm systems wherein the receiving unit of the system automatically provides an indication as to the signal strength of a received signal by measuring the signal strength of the signal and comparing it against a threshold.
Most radio frequency (RF) wireless security systems available today, such as those manufactured by ADEMCO, generally employ a multiplicity of transmitter products which transmit information to a common receiver/controller. The information transmitted typically describes the state of various transducers associated with each transmitter, such as smoke, motion, breaking glass, shock and vibration detectors; door, window and floor mat switches, etc. Each signal has a unique identification code embedded in its data message, which serves to identify to the system controller which particular transmitting device has sent that message.
Wireless security systems operating in residential and commercial buildings are often relied upon for Safety of Life applications. Many national regulatory agencies place stringent requirements on the operation of these types of systems, in the USA, Underwriter's Laboratories issues specifications, in the UK, British Standards apply, and in most of Europe, CENELEC harmonized norms set the specifications. In particular, it is becoming more common for these specifications to require that the received signal strengths from all sensor transmitters should be recorded at the time of installation such that at a later time of a periodic building inspection, an inspector can compare received signal strengths with those which were recorded at the time of original installation and relocate transmitters if necessary. In this set up, the inspector is relied upon for determining if there is a signal below margin. This may impose human error. Also in an environment where there are many changes, the inspector may not monitor the signal at a time when the signal is below margin.
In commercial buildings, particularly warehouses where material is continuously moved around, there may be sudden and significant changes in radio propagation due to a blocked signal path. Systems which are currently available, such as Ademco's 5800 system, determine this trouble condition by checking that each transmitter has sent a periodic check-in message to the receiver/controller. In these systems, if several check-in messages have not been received, then a supervision failure is indicated. Thus, the currently available systems will detect a transmitter's failure due to a changed environment or transmitter failure. However, these systems give no indication that a system's performance has been reduced and is in a marginal condition, i.e., the check-in messages still get through, but with poor margin. If an alarm condition should occur in this marginal state, then there is a risk that the message may not get through to the receiver/controller system.
It is therefore an object of the present invention to provide a communications system suitable for use with an alarm system which provides means for constantly measuring the signal strength of received messages (whether checkin or alarm) and automatically detecting a change in signal strength for each system transmitter.
It is a further object of the present invention to be able to measure and store signal information, such as the signal strength, of each transmitter during an install mode.
It is a further object of the present invention to provide a communications system suitable for use with an alarm system which provides an indication of a marginal condition at about the time that the signal margin is reduced.